1. Field of the invention
This invention relates to a device for quickly heating a heatable element which has a high thermal lag, at least initially. The term "thermal lag," as used herein, refers to the fact that when electrical power is applied to a heatable element that is thermally loaded by its own or by neighboring material that absorbs heat, the rise in temperature lags behind the temperature that would exist in the absence of such material. An electrical iron, a soldering iron or an electric stove element are examples of this. It is known that considerable time must elapse before the device comes up to its desired temperature. The thermal load may change during the heating process. For example, the glowplug of a model airplane engine may be "drowned" by the introduction of priming fuel, as will be explained later. As soon as the priming fuel has been evaporated, however, the thermal load is greatly reduced and the glowplug temperature now rises rapidly. This invention is applicable both to the case where the thermal load, and thus the lag, is essentially constant as well as to the case where the thermal lag is high initially.
2. Description of the Prior Art
Small internal combustion engines that are commonly used to power model airplanes are usually equipped with a glowplug to provide ignition. The glowplug contains a filament which is heated by a battery just before the engine is to be started. After the engine has started the battery is disconnected from the glowplug and from then on the combustion of fuel within the engine keeps the glowplug hot enough to cause ignition.
In order to start such a small engine, it is usually necessary to prime it by injecting liquid fuel into the combustion chamber, either through the exhaust port or via the carburator, as is well known. This act frequently drowns the plug and prevents it from attaining the necessary temperature. The reason is that the heat conductivity, heat capacity, and heat of vaporization of the liquid fuel are so high that heat is conducted away from the glowplug filament very easily; so easily that when heated by the usual battery source of energy the heating element of the glowplug fails to get hot, the liquid fuel is evaporated slowly at best, and the engine will not start. Thus, an engine wet enough to run if it had ignition is likely not to have any. One dry enough to ensure ignition, i.e., in which the glowplug is not drowned, will not willingly start. This problem is particularly acute in cold weather because the fuel is then hard to vaporize. It is also a problem when the engine is used in an inverted position, since in that case the glowplug is at the bottom of the combustion chamber where liquid fuel will naturally gather. However, the problem is not limited to cold weather or to inverted engines, by any means.
It might at first appear that this problem could be solved simply by using a higher starting voltage so as to apply more power to the glowplug. However, if this should be done the added heat would, indeed, evaporate the liquid fuel and dry out the filament more quickly, but as soon as this happened the temperature of the filament would rise above that for which it had been designed, and it would burn out. It is impractical, therefore, to apply to the glowplug a fixed voltage in excess of the voltage for which it has been designed. Accordingly, starting the engine remains a frustrating process of skill, patience and time, with success rewarding those who hit the lucky amount of priming which starts the engine without drowning the glowplug.